The development of cultured embryonic myoblasts occurs in two distinct phases. The first is purely proliferative, generating a dense population of cells indistinquishable from their progenitors. These cells then undergo rapid transition. The cells divide more slowly and fuse to form long multinucleated fibers which now synthesize muscle-specific myosin and CPK at rapid rates. Although fusion and myosin synthesis occur concomitantly, they are not obligatorily coupled. When fusion is blocked in low Ca medium, muscle-specific synthesis is initiated in mononucleated "myocytes". In common, both myocyte and muscle fiber nuclei withdraw from the cell cycle during a protracted G1 which, in turn, is a response to the culture milieu. Since the "decision" to differentiate or to reenter the cell cycle occurs in G1 when the events associated with differentiation occur, using M-synchronized myoblasts cultured in media which prevents fusion but protracts G1. Using inhibitors of macromolecular synthesis to interrupt or delay entry into S and ts mutants of G1 (in myoblast lines) as finer probes, we plan to examine alternative switching mechanisms which might direct into the differentiative mode rather than entry into S. The block to DNA synthesis and the stability of differentiation will also be examined by transplanting muscle cell nuclei into Xenopus eggs and oocytes, environments primed to support DNA synthesis and coupled transcription-translation, respectively.